Start with some reading;

https://www.autocodes.com/p1131_2003_ford_f150.html

Ford P0171 & P0174 Lean Codes

Engine Vacuum Leaks

List any other codes you have.

 

 

Possible causes - 1131

• Faulty Upstream Oxygen Sensor Bank 1
• Fuel pressure high or low
• MAF sensor dirty/defective
• Vacuum leak on engine
• Leaking fuel injector or fuel pressure regulator
• Faulty Powertrain Control Module (PCM)

You would think the MAF and PCM would effect BOTH sides of the motor.. Same with a vacuum leak........ the 171 code says the problem is on the Passenger side.


I would clean the MAF.... but look at injector pluggage / leaking problems on that bank.

 

The first thing likely with a red converter is raw gas is burning in the cat.
This has likely ruined the cat and needs replacement.
It will also take out the Ox sensor from the excess heat. They are not made to run at those temperatures of over 900 degrees.
Next, one or more cylinders have lost ignition such that it's fuel injector is dumping the fuel the cat is burning.
Very likely the burned Cat 'melted' is restricting the exhaust from that bank.
You have to first clear the misfire.
Then use a Scanner to look at the performance of the front and rear Ox sensors to see if they are working properly or need replacement ... or .... indicate the Cat needs replacement.
Normally if the Cat is burned, contaminated or broken up, code 420/421 will set.
You have a more complex diagnostics to see and clear all the faults that have stacked up.
It's not so simple overall but you must recognize the implications of the situation and look at each one separately.
Good luck.